1. Field of the Invention
The present invention relates to an interferometer to be used for measuring a surface shape of an optical member or the like.
2. Description of the Prior Art
Interferometers are now widely used as highly accurate means for measuring a surface shape of an optical member or the like. In a typical interferometer, coherent light emitted from a light source is introduced through a beam splitter to a reference surface and a subject surface to be measured, each of which reflects the incident light to the beam splitter. The reflected lights by the reference surface and the subject surface are then introduced to an imaging optical system through the beam splitter. The imaging optical system causes the reflected lights to interfere with each other to form an interference fringe, which is observed by use of a detection optical system. Where the subject surface has an undulation, the optical path length of the incident and reflected light to and from the subject surface is changed to cause an optical phase shift corresponding to the undulation. Therefore, the subject surface profile can be measured by observing the interference fringe formed through interference between the two reflected lights. In this type of interferometer, it is necessary, prior to the observation of the interference fringe, to perform alignment for accurately determining the position and orientation of the subject surface. In aligning the subject surface, spots are formed by reflected lights from the reference and subject surfaces by use of an alignment optical system, and then coarse alignment is effected by making those two spots coincide with each other. Alternatively, coarse alignment is attained by placing the reflected light from the subject surface an alignment mark. Then, after the alignment optical system is replaced by a measurement optical system, fine alignment is performed while the interference fringe is observed. Conventionally, the alignment optical system is provided entirely separately from the measurement optical system which deals with incident and reflected lights to and from the both surfaces, as well as from the imaging optical system for causing the reflected lights from both surfaces to interfere with each other to be imaged into the interference fringe. As a result, the overall size of the interferometer is large, and a cumbersome operation is needed to switch between the aligning mode and the measuring mode. In addition, since the magnification of the alignment optical system is conventionally fixed, the alignment accuracy cannot be improved beyond a certain limit.
In recent years, with the use of light sources of shorter wavelength, requirements for the profile irregularity of optical members have become stricter. For example, in the case of an optical element for use with X-rays, the required accuracy is as high as .lambda./100, where .lambda. is the wavelength of light used in an interferometer, e.g., 633 nm. In order to conduct a measurement on the profile irregularity of so strict a level, an interferometer should also be highly accurate. However, an interferometer has aberrations, which cannot be eliminated completely. To solve this problem, as is customarily practiced, a standard surface which has been examined and assured to be very accurate is measured using an interferometer, and resulting measurement data is subtracted from measurement data obtained with an actual subject surface. Thus, aberrations of the interferometer itself can be eliminated. In practicing this method, the profile irregularity of the standard surface must be sufficiently superior to that of the actual subject surface. However, since it is very difficult to manufacture a highly accurate standard surface, it is an established procedure to use the same standard surface commonly for measurements of subject surfaces having different curvatures. In this case, when an actual subject surface is measured, focusing needs to be performed again after aberration data of the interferometer itself has been obtained by using the standard surface. Since this refocusing operation changes the size of the interference fringe, it is impossible to correctly compensate for the aberrations.
It is common to use a polarizing beam splitter in order to effectively utilize light emitted from a light source. In this case, a quarter-wave plate is inserted in an optical path so that the linearly polarized incident light entering into the quarter-wave plate and the linearly polarized reflected light exiting from the plate are perpendicular in vibration direction with each other. In conventional interferometers, the quarter-wave plate is disposed directly behind the polarizing beam splitter where the light assumes a parallel form. Therefore, light reflected by a lens surface that is located directly behind the quarter-wave plate has the same polarization angle as measurement light, i.e., reflected light coming from a reference surface or a subject surface. Thus, the reflected light as produced by the lens surface located right behind the quarter-wave plate returns to the polarizing beam splitter together with the measurement light and is introduced to the imaging optical system, so as to become noise superimposed on the measurement light. This noise is a factor of causing an error in measurement results.